1. Field of the Invention
The invention concerns the domain of crystal scintillators, in particular those which use a crystal of alkaline halogenides such as NaI(1) and more especially those which exhibit a good resistance to shocks and extreme temperatures.
2. Discussion of the Related Art
In order to detect invisible radiation, light of very short wave length (.gamma. rays) or electronic radiation, crystals are currently used which transform this radiation into photons of longer wave length which can be detected and measured (in general by counting) with traditional photo-multipliers.
These appliances, called scintillators, are used in particular for oil prospecting where, in association with drilling tools, they permit the gathering of information on the ground crossed.
These very severe conditions of use and in particular the high temperatures and very violent shocks to which the scintillators are subjected necessitate designs of these appliances which are adapted to permit them to suffer these temperatures and these shocks without deterioration, and in particular without their response characteristics being modified.
The usual scintillators consist of four principal components: a case, a window, a crystal and a reflector. The case is cylindrical and made of stainless steel with an opening at each end. One end is closed by a glass window, hooped or glued in the opening. The photo-multiplier which is intended to detect the photons emitted in the crystal is placed against this window. The crystal itself is made of a material which scintillates under the action of ionizing radiation, in general a halogenide of alkaline or alkaline earth metals, preferably an iodide such as, for example, sodium iodide doped with thallium NaI(T1). Its shape is also cylindrical; its surfaces are polished. One of the plane ends is generally optically connected to the window, for example, by means of a silicon resin and on its other faces--the cylindrical side face and the second plane end--it is equipped with reflectors which collect the light emitted and rediffuse it towards the window and the photo-multiplier situated behind it. The end of the cylinder opposite the window is tightly closed.
In scintillators designed specially to stand up to high temperatures and shocks, such as for example in the one which is described in the U.K. patent application GB 1 547 341, there has been provided behind the crystal a deformable elastic element which maintains optical contact between the crystal and the window, even when a shock acting on the mass of the crystal tends to separate it from the window.
Concerning the cylindrical side face of the crystal, it is best here also to maintain a close contact between this surface and the reflector. The British patent application GB 2 084 169 provides for a very thin layer made of a plastic strip of polytetrafluoroethylene (PTFE), and this serves as a lubricant. Between this strip and the internal wall of the metal cylinder the space is filled with a fine powder of alumina or magnesium oxide. In addition, at the rear, the elastic system is reinforced by spring washers which press strongly against the crystal. The lateral protection system described in the U.K. patent application GB 1 547 341 for its part, provides for a connection between the inside of the metal cylinder and the crystal which is constituted by a silicon elastomer. This transparent sleeve, possibly filled with powder, contains numerous excrescences in contact with the crystal and, between these, a filling with a powder such as alumina.
In another proposed solution, that of U.S. Pat. No. 4,900,937, it is specified to allow a greater longitudinal mobility of the crystal so as to permit, in the event of a shock, a material separation between the latter and the material which normally serves as an optical connection with the window, optical contact being automatically restored thanks to the recall springs as soon as the acceleration disappears.
The above systems taken in isolation or possibly in combination generally give satisfaction in the desired temperature range (up to 150.degree. C.) and for accelerations up to 150 times that of gravity (.ltoreq.150 g). However, when the shocks are more violent, for example they cause an acceleration of 500 g, the scintillators cannot stand up to it any longer and breakaways are found either at the window-optical connection or the optical connection-crystal interfaces or on the other faces of the crystal in contact with the reflector. Such breakaways modify the optical characteristics of the scintillator, and the response of the photo-multiplier corresponding to a given signal is no longer the same.